Acute administration of the potent hallucinogen LSD results in an activation of striatal tyrosine hydroxylase (to 150% of control). The potent neuroleptic haloperidol produces a similar alteration in enzyme activity and when given together the effects of LSD and haloperidol are additive. LSD, like most dopamine receptor antagonists, can activate tyrosine hydroxylase and this activation may be dependent on cAMP.